🤚🏻Factors Affecting Enzyme Activity and Enzyme Inhibitors

From Field to Lab — When Enzymes Stop Working

In the previous lesson, we explored enzyme structure — what enzymes are, how the lock-and-key and induced-fit models explain specificity, and how cofactors and coenzymes assist catalysis. Now we turn to the practical question: what controls how fast an enzyme works?

A farmer stores wheat grain at 12% moisture and it remains viable for years — the enzymes inside are inactive because there is not enough water. Add water during sowing, and enzymes spring to life, breaking down starch into sugars that fuel germination. Now consider the opposite problem: a seed lot stored in a hot, humid warehouse. The high temperature and moisture activate enzymes prematurely, the seed “respires away” its food reserves, and germination rates plummet.

These real-world examples demonstrate that enzyme activity depends critically on substrate concentration, temperature, pH, hydration, and the presence of inhibitors. Understanding these factors is essential for both agriculture and biochemistry exams.

This lesson covers:

1. Factors affecting enzyme activity — substrate concentration, enzyme concentration, temperature (Q₁₀), pH, hydration, and end-product effects
2. Enzyme inhibitors — competitive vs non-competitive (reversible and irreversible), feedback inhibition
3. Agricultural and medical applications — seed storage, sulpha drugs, cyanide poisoning

All sections are high-yield for IBPS AFO and ICAR exams.

---

Factors Affecting Enzyme Activity

Enzyme activity is not constant — it varies with the physical and chemical environment. Six major factors determine how fast an enzyme catalyses its reaction. Each has direct agricultural relevance, from seed storage to soil enzyme activity.

1. Substrate Concentration

• Increasing substrate concentration initially increases reaction rate
• When all enzyme active sites are occupied, the rate reaches a plateau called Vmax (maximum velocity)
• Beyond Vmax, adding more substrate has no effect — the enzyme is saturated

---

2. Enzyme Concentration

• Increasing enzyme concentration increases reaction rate — until substrate becomes the limiting factor
• This mirrors the relationship with substrate concentration: whichever is in limiting quantity controls the rate

---

3. Temperature

• Q₁₀ (temperature coefficient) = rate at (T+10°C) / rate at T°C = 2 for most enzymes
• Denaturation occurs because high temperatures disrupt hydrogen bonds and weak interactions that maintain the enzyme’s 3D structure

---

4. pH (Hydrogen Ion Concentration)

• Ideal pH for most enzymes: 7–7.5
• Exceptions exist: Pepsin works best at pH 1.5–3.0 (stomach acid); Trypsin works at high pH (alkaline intestine)
• pH changes alter the ionisation state of amino acids at the active site, affecting substrate binding

---

5. Hydration (Water Content)

• In dry seeds, water is too low for enzymatic activity → seeds remain dormant
• Adding water → enzymes become active → seed germinates
• Water serves as both solvent for enzyme-substrate interactions and a direct reactant in hydrolysis reactions

---

6. End Product Concentration

• Enzyme reactions are reversible (mass action principle)
• Accumulation of end products increases the rate of the reverse reaction
• In living cells, end products are continuously removed by subsequent metabolic reactions, keeping the forward reaction going

---

Enzyme Inhibitors

While the previous section covered physical factors (temperature, pH, water), enzymes can also be slowed or stopped by specific molecules that interfere with catalysis. These inhibitors are central to drug design (sulpha drugs, antibiotics), pesticide development, and the cell’s own metabolic self-regulation.

Enzyme inhibitors are molecules that reduce or prevent enzyme activity. The key distinction is where the inhibitor binds and whether the inhibition can be reversed.

Comparison of Inhibitor Types

---

Competitive Inhibition

• Inhibitor is structurally similar to the substrate (substrate analogue)
• Competes for the same active site
• Can be overcome by increasing substrate concentration
• Example: Malonate inhibits Succinic dehydrogenase (malonate mimics succinate)

---

Non-competitive Inhibition (Irreversible)

• Inhibitor binds at a different site from the active site
• Destroys the sulfhydryl (S-H) group of the enzyme
• Cannot be reversed by adding more substrate
• Examples: CN⁻ (cyanide), CO, toxic metals

---

Non-competitive Inhibition (Reversible) — Feedback Inhibition

• Inhibitor binds at the allosteric site reversibly
• The end product of a metabolic pathway inhibits an earlier enzyme in the pathway
• Also called retro inhibition or feedback inhibition
• Example: Glucose-6-phosphate inhibits hexokinase

---

Comparison Table — Competitive vs Non-competitive Inhibition

---

Summary Table — Key Facts at a Glance

Summary Cheat Sheet